The present invention relates to a method for regulating a predetermined modifiable brake pressure in the wheel brakes of a brake system, wherein input quantities determining the brake pressure in the individual wheel brakes are evaluated and correcting variables of hydraulic valves are defined in a control and/or data processing system.
Vacuum brake boosters require a vacuum supplied by the engine for boosting the pedal force to be generated by the driver. Depending on the engine, even relatively low pedal forces allow reaching a condition where further increase of the force applied to the actuating unit is possible only by an increase in the pedal force because the vacuum brake booster has reached the maximum possible boosting force. This condition is referred to as the point of maximum boosting of the booster. Braking operations that take place in excess of the point of maximum boosting of the vacuum brake booster place high demands on the driver in terms of the pedal force to be generated. For this reason, brake systems (OHB-V) are employed at an increasing rate supporting the driver by means of an active hydraulic brake pressure build-up. Brake systems of this type generally include a motor-and-pump assembly and a hydraulic unit being controlled by an electronic control such as an electronic stability program (ESP), antilock brake system (ABS), traction control system (TCS), and similar systems, to comply with the desired brake force boosting.
When pressure is built up in the wheel brakes by means of hydraulic boosting beyond the point of maximum boosting of the vacuum brake booster, the (analogized) separating valves are closed, and the hydraulic pump delivers brake fluid from the tandem master cylinder (TMC) into the wheel brakes. In the pressure reduction phase, it is required to discharge the additionally generated pressure into the tandem master cylinder by way of the separating valves in a manner comfortable with respect to pedal feeling and braking effect. This action usually requires two additional pressure sensors at the wheel brakes, one per brake circuit, in order to determine the wheel pressure. The result is a higher requirement of components causing an increase in the system's costs.
During failed boost brake applies, the brake pedal forces required to enter OHB are very high. This is due to the increase of the booster cut-in force at zero vacuum, and the inaccuracies of the master cylinder pressure transducer at low pressures. In prior systems, the driver had to generate enough pedal force to overcome the cut-in force of the booster, before pressure could be built in the master cylinder. Next, the driver had to generate additional force, before the master cylinder pressure transducer would output a reliable signal at which point OHB would start to generate pressure at the wheels. This pedal force can reach 250 Newtons or higher.
In view of the above, an object of the invention is to provide a method for regulating a predetermined variable brake pressure in the wheel brakes of a brake system, wherein when a very low booster vacuum level is detected the OHB system is used to command an increase in wheel pressure.